The present invention relates to an unconsumable electrode welding apparatus such as a hot-wire TIG (Tungsten Inert Gas) welding apparatus, or more in particular to an AC TIG welding apparatus using a hot wire suitable for efficient AC arc welding of aluminum or the like.
In welding aluminum or an aluminum alloy, the AC TIG welding process is usually used for a thin plate less than about 5 mm in thickness and the MIG (Metal Inert Gas) welding process for thicker plates. The MIG welding process, which is high in welding rate and small in heat input as compared with the TIG welding process, may have a narrower heat-affected zone with a smaller softened zone and thermal strain. Nevertheless, this welding process has the disadvantage that a blowhole is liable to develop and the corrosion resistance of the deposited metal is comparatively low. Further, this process is not applicable to the welding of a thin plate in view of arc stability and bead formation.
The TIG welding process, on the other hand, in spite of a comparatively low welding rate and a large heat input resulting in a wide heat-affected zone with an increased softened zone and thermal strain, has the advantages of superior bead shape, less blowholes and high corrosion resistance of the deposited metal. A large arc current cannot be used for thin plates due to the problem of melt down, and therefore it is desirable to improve the welding rate without increasing the arc current to reduce the softened zone and thermal strain.
FIG. 1 is a diagram schematically showing a welding apparatus conventionally used for automatic TIG welding of aluminum or aluminum alloys.
As shown in FIG. 1, an AC arc 7 is formed with a base metal 6 by an AC arc power supply 16, a TIG torch 4 and tungsten electrode 5, and a filler wire 9 is guided to the weld zone through a conduit 10 by use of a wire supply unit 8. The weld zone is thus fused by the arc 7 thereby to form a deposited bead 13. A water cooling system and a shield gas system for the torch 4 are not shown. An inert gas such as argon gas is caused to flow out of the torch 4 to shield the weld zone.
In the TIG welding of aluminum, in which an oxide film on the surface of the base metal 6 would make the welding process difficult, the AC arc 7 is used to allow a period to form a negative pole of the arc at the base metal 6 and an oxide in the surface of the base metal 6 is broken by the impact of ions entering the negative pole in what is called "the cleaning action". The TIG welding process conducted while removing an oxide film this way is the most common practice for welding aluminum. On the other hand, the TIG welding of copper or copper alloys commonly uses a DC electrode negative polarity. In welding aluminum bronze or beryllium bronze, however, the welding process is facilitated by the use of an AC arc producing the cleaning action. In the conventional AC TIG arc welding process, a deposited metal is generally formed by melting with the arc a filler wire made of what is called a "cold wire" which is not heated by conduction of electricity.
An AC TIG arc power supply for producing a square wave AC current by switching a DC current is often used as a power supply for arc. An example of such an arc current designated by i.sub.A is shown in FIG. 2. The period of this waveform is set to 7 ms, of which the term of, say, 1 ms of the half wave of the electrode positive polarity, forming a negative pole of the base metal, is adjusted to approximately 1.5 ms length thereby regulating the degree of the cleaning action of the arc.
The use of a hot wire in AC TIG arc welding process increases the wire melting rate and therefore is expected to improve the efficiency of the welding work. The welding process with hot wire has not been used in the past for aluminum or aluminum alloys, however, probably because the small specific resistivity of these metals requires a large current in Joule heating, often causing a excessive magnetic blow of the arc.
The hot wire TIG welding process is often used in combination with the DC TIG welding process for carbon steel, stainless steel or the like. This process uses as a wire heating current a DC or AC (Reference: Hot Wire; J. F. Saenger; Welding and Metal Fabrication, June 1971, or a DC pulse (U.S. Pat. No. 4,614,856) for protection against arc magnetic blow.
An experiment conducted by the inventors on a combination of the AC TIG arc with a wire heating power supply of this type shows that the arc is so unstable that the likelihood of arc interruptions poses a bottleneck to the applicability, especially to aluminum.
A mere combination of a hot-wire heating power supply with the AC TIG arc poses the problem of unstable arc and the resulting inapplicability of the hot-wire welding. A study of the cause of this problem in aluminum welding has revealed that when the unconsumable electrode is positive producing the cleaning effect, the arc wanders seeking an oxide in the base metal surface (Reference: Welding Arc Phenomena; Kohei Ando, et al.; July 1962, p. 141 to 144), and if the wire is supplied with current during this time, the magnetic blow of the arc due to the magnetic field generated by the wire current aggravates the arc instability with the result that the arc is liable to be interrupted.